1. Field of the Invention
This invention relates to a method and an apparatus for making lead-acid batteries and, in particular, to an improved method and apparatus for forming joints of plate lugs and post straps of lead-acid storage batteries.
2. Description of the Related Art
The making of lead-acid storage batteries usually involves plate preparing process and assembling process. The latter process involves plate/separator stacking, joint forming necessary to build up cell groups, and group insertion into vessels or containers. The present invention relates to a method and an apparatus for mechanically and electrically joining positive and negative plates to respective positive and negative posts, to compose a group of positive plates, negative plates and separators. There are two methods in general to electrically and mechanically join plates with posts. A first method is a burn-together method that is applied to a plates of upright position. Lugs of positive and negative plates are inserted in respective strap-forming molds having comb-like slits, and positive and negative posts are put in position in each mold. Then, plate lugs and a bottom part of the post are melted by a torch using lead alloy solder to joint the lugs and post. A second method is so-called "cast-on-strap" method (COS) in which lugs of inverted plate groups are inserted into molten lead or lead alloy filled in a mold of a necessary shape of cavity, and after cooling, the group is removed from the mold. The latter method simultaneously provides posts when forming straps, and the process is easy to be automated. An early embodiment of the latter was described in U.S. Pat. No. 3,087,005, and many modifications and improvements have been publicized after the issue.
In the COS method, the surface of the lugs should be wetted with the molten lead to form a perfect joint between each other. This could only be attained by keeping the temperatures and relating conditions of lugs and molten lead in a mold within strictly narrow range even though various treatments are applied on the surfaces of lugs. To realize the satisfactory operating condition, various improvements were proposed on molds and relating apparatus. Japanese Patent Kokai H01-195662 proposes heat insulators between "cast-on" mold and molten lead supply channels. The supply channels have heaters in them and the mold equips a cooling device to control the temperature condition, and the mold is made to have a big heat capacity to minimize local temperature difference and the temperature of the molten lead must be kept sufficiently high. An example of the operating condition by using the similar apparatus is: Molten lead of 500.degree. C. is introduced in the mold of 240.degree. C. When the temperature of lead goes down to 380.degree. C., the lugs of a group are quickly inserted into the molten lead and the cooling device is actuated. After the lead solidifies, the group is removed from the mold. In this operation, molten lead is significantly oxidized, and the timing of lug insertion is very narrow, while cooling of the mold having a big heat capacity takes time. The mold construction is also complicated and it takes much time in changeover.
To improve these drawbacks, Japanese patent Kokai H05-03028 proposes a manufacturing method including a process in which sprayed molds are dipped in a pot of molten lead to homogeneously heat them. The method affords a stable operation and is suitable for a production in quantity but it requires a relatively large energy consumption, and changeover is also not easy.
Japanese Patent Kokai H07-122259 discloses a simplified mold and shortened heating and cooling times. This mold, although simple, is still heavy in weight and large in size compared with those of straps formed by it, and this mold is not suitable for an automated facility. Japanese Patent Kokai H08-212999 discloses a COS method in which mold thickness is thinner at around a part, or the whole of its cavity than that of straps to be formed, and a heating and cooling system including the utilization of Joule's heat by an electric current across the mold. In this case, the mold must have a thicker part in which Joule's heat does not much generate, and distortion of a whole mold is serious because of the heat cycle to the mold of a part of a mold of extremely different thickness. Also, this method is hard to apply to simultaneously form big post when lugs are welded together as a strap.